Acoustic duct with asymmetric acoustical treatment

Abstract

Asymmetric or peripherally discontinuous acoustic linings for absorbing sound radiating within acoustic ducts, when properly located in the peripheral or circumferential direction, alter the directivity of sound emitted from an end of the duct to provide preferential enhanced suppression in a predetermined general direction. In a jet engine fan duct, treating the upper half produces approximately the same noise suppression on the ground as a fully treated duct. Another application is side lobe suppression. Two different treatments can be used providing the asymmetric lining is the better sound absorber.

Description

BACKGROUND OF THE INVENTION

This invention relates to sound-absorbing acoustic ducts, and more particularly to an acoustic duct with asymmetric or peripherally discontinuous acoustical treatment. As a typical application, the invention relates to an asymmetric noise suppressing acoustic lining for the inlet of an aircraft jet engine fan.

Sound-absorbing acoustical material used as a lining in acoustic ducts is ordinarily applied symmetrically in the axial or longitudinal direction. That is, the acoustical material is circumferentially or peripherally continuous at any given axial or longitudinal location. By way of example, the sound absorbing linings at the inlet air passage of a jet engine fan in an airplane are applied over a full 360.degree. of the internal surface of the fan cowling or casing. This is illustrated in U.S. Pat. No. 3,542,152 to A. P. Adamson, G. D. Oxx, Jr., and W. R. Morgan, assigned to the same assignee as this invention, in which the acoustical material is a honeycomb-type panel with tuned resonant cavity structures for the absorption of broadband noise. Provision is made for drainage of ingested liquid both in the sound-absorbing panel itself and in the cowling. Although advantageous for this application, the improved asymmetric acoustical treatment technique has utility in numerous other applications such as silencers for industrial gas turbines, and modifying the directivity patterns of acoustical horns.

SUMMARY OF THE INVENTION

It has been found that an asymmetric or peripherally discontinuous acoustic lining for absorbing sound radiating within an acoustic duct, depending upon its location and extent in the peripheral direction, has the capability of selectively altering the directivity of sound emitted from an end of the duct. The altered directivity of radiated sound or noise is employed to provide preferential enhanced suppression of sound in a preselected angular sector of the directivity pattern. In the preferred embodiment, fan noise produced in an aircraft jet engine fan duct is selectively suppressed in the sector generally beneath the fan duct inlet by treating only the upper portion of the inner surface of the duct. A fibrous acoustic material with a higher sound absorption coefficient can be used, and for a 180.degree. semicylindrical acoustic lining, for example, the amount of noise suppression as to a ground observer is approximately the same as for the full 360.degree. treatment. The advantage is thus a reduction of required acoustic material or weight, or conversely an increased effect for the same amount of treatment. It is believed that the theoretical explanation relates to the better reflection of sound by the untreated lower inner surface of the rigid or hard-walled duct.

As a modification of the invention, a second acoustic lining is used to treat all or a portion of the remaining inner surface of the fan or other acoustic duct. To obtain an asymmetrical directivity pattern, the first-mentioned asymmetric lining has a higher sound absorption coefficient than the second lining. Another embodiment is an asymmetrically treated acoustical duct for side lobe suppression, which includes an acoustic lining with a pair of opposing sound-absorbing strips, each of which effects selective suppression of the opposite side lobe in the directivity pattern. These are illustrative of the many variations of the asymmetric placement of acoustic material in acoustic ducts for a desired preferential effect, and of the variety of possible applications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic side elevational view, partially in longitudinal cross section, of the forward portion of a ducted fan type aircraft jet engine having a fan duct provided with an asymmetric semicylindrical acoustic lining according to the teaching of the invention;

FIG. 2 is a vertical cross-sectional view of only the inlet fan duct or casing taken on the line 2--2 of FIG. 1 and showing the 180.degree. asymmetric acoustical treatment for preferential noise suppression as to a ground observer;

FIG. 3 shows three typical experimentally obtained directivity patterns for a cylindrical jet engine fan duct for the cases when the duct is untreated, has 360.degree. acoustical treatment, and has a 180.degree. acoustical treatment;

FIG. 4 is a cross section similar to FIG. 2 illustrating another aspect of the invention using two different acoustic lining materials for optimum economic and noise suppression effect; and

FIG. 5 is a cross section through an acoustic duct with asymmetric treatment for side lobe suppression.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Although the invention has general applicability to acoustic ducts used for noise and sound suppression, the application discussed in detail with regard to FIGS. 1-4 is an asymmetric or circumferentially discontinuous acoustic lining for the inlet duct of an aircraft jet engine fan to reduce the noise level heard at the ground during take-off and landing. In FIG. 1, there is shown generally at 10 a ducted fan type turbojet engine having an annular streamlined fan duct air passage 11 formed by an annular cowling or fan casing 12 of streamlined cross section and a suitable engine nacelle structure 13 projecting within the cowling 12. The nacelle structure 13, of which only the forward portion is shown here in outline, houses a suitable compressor, combustor, and turbomachinery for, as part of its functions, driving a fan 14 disposed in the air passage 11 between the forward end of the cowling 12 and the nacelle structure 13. The fan 14 drives inlet air axially through the primary air passage 11 to provide propulsive thrust to the engine as well as to supply air to the compressor through a second, inner air passage 15. The major part of the fan flow exits through an annular exhaust nozzle opening 16 formed by the inner surface of the cowling 12 and the outer surface of a gas generator pod casing 17. The compressor inlet air passage 15 is formed between the inner casing 17 and the forwardly projecting, tear-shaped, fan mounting and drive structure 18. The outer surface of casing 17 is suitably lined with the honeycombed, resonant chamber sound-absorbing panels 19 previously discussed. For further information on this type of turbofan jet engine, reference may be made to U.S. Pat. No. 3,540,682 to C. G. Dibble and D. F. Howard, assigned to the same assignee as this invention.

As best shown in FIG. 2, the inside surface of the cowling 12 has an attached, asymmetrical semicylindrical acoustic lining 20 which, in this embodiment of the invention, covers only the top half of the inner surface. This structure will hereafter be referred to as an asymmetrically treated fan duct. By using the 180.degree. sound-absorbing acoustical treatment on the upper half of the fan duct, the suppression of noise as to a ground observer is almost the same, or approximately the same, as if the prior known 360.degree. acoustical treatment were used. To further explain this, the directivity of noise emitted by the fan 14 is altered such that there is preferential or enhanced suppression of the noise pressure level in a selected direction, in this case generally beneath the noise source. The fan 14 produces broadband noise, and is the major source of jet engine noise. A variety of acoustic lining materials can be used, including the honeycombed, resonant chamber sound-absorbing panels shown in the previously mentioned U.S. Pat. No. 3,542,152, but it is preferred to employ a fibrous acoustic material with a higher sound absorption coefficient. Since the acoustic lining 20 covers only the top half of the fan duct, the need to use a material which provides for drainage of ingested liquids is diminished if not substantially eliminated. Among the suitable fibrous acoustic materials are fiberglass, stainless steel wool, and mineral wool for high temperature portions of the duct. The advantage of the 50 percent or asymmetrically treated fan duct is evident, since practically the same noise suppression effect is obtained with only half the cost and weight as the previously full 360.degree. treatment. In addition to minimizing the treatment weight, the cost per square foot of the above and other fibrous acoustic materials may be less than that of the honeycombed, resonant chamber sound-absorbing panels. Alternatively, for the same treatment weight there is an increased noise suppression effect. Further, there may be lower air flow losses in the fan duct due to the improved flow over the smooth, untreated duct surfaces.

The altered, asymmetric directivity pattern of noise radiating from the inlet end of a semicylindrically, 50 percent treated fan duct is shown in FIG. 3. The upper half of the experimental fan duct 21 is the acoustically treated semicylinder, while the bottom half is the untreated semicylinder. Measurements were made in an anechoic chamber using a one-third scale model of the General Electric CF6 jet engine fan run at 90 percent design speed. To provide a basis for comparison, the symmetrical directivity patterns for an untreated fan duct and a 360.degree. fully treated fan duct using the same acoustic material are illustrated respectively in dashed and dotted lines. Immediately beneath the duct inlet, the reduction in sound pressure level produced by the full acoustical treatment is about 8 decibels. The asymmetrical directivity pattern obtained by using the new 180.degree. treatment is shown in full lines. It will be observed that over a large angular sector generally beneath and in front of the duct inlet, the amount of noise suppression using the 180.degree. treatment is approximately the same as for the full 360.degree. treatment. For the data taken, the noise reduction with the 180.degree. treatment as compared to full treatment is almost the same over the sector from 50.degree. to 110.degree. measured downwardly with reference to the forwardly projected duct axis. The noise suppression effect immediately in front of the duct inlet is not as favorable as that obtained by use of the full treatment, but there is less concern about noise suppression in these spatial regions since the primary objective of the acoustical treatment is the reduction of noise heard by human beings at ground level. Immediately above the duct inlet the noise suppression effect of the treatment is about half that of the full 360.degree. treatment. The redirection of noise obtained by use of the asymmetrical acoustical treatment, shown here for an axisymmetrical noise source, is independent of the type of acoustic material employed, and it is understood that the circumferential location of the asymmetric treatment determines the general direction at which the preferential noise suppression is obtained. Thus, for the case of reducing fan noise for another application, when the bottom half of the fan duct is treated rather than the top half, the preferential, enhanced noise suppression is obtained above the duct inlet rather than below.

Although the theoretical explanation for the altered directivity of sound radiating from an asymmetrically treated fan duct or other acoustic duct is not known with certainty, it is believed it can be explained in terms of the reflection of sound by the internal surfaces of the duct. Broadband noise emitted by the axisymmetrically located fan radiates in all directions, and some of the sound waves are incident upon the treated semicylindrical duct surface, others are incident upon the untreated semicylindrical surface, and a portion radiates directly out the end of the fan duct. Sound striking the treated upper half is partially absorbed and partially reflected, while that incident on the untreated lower half of the duct, which is lined with smooth sheet metal panels, is almost totally reflected. Some of the sound reflected off the untreated lower half is, in turn, incident upon the treated upper half where it is partially absorbed. Conversely, some of the sound reflected from the treated upper half is radiated toward the lower half, where it is again reflected to the sound-absorbing upper half. Eventually, the unabsorbed sound energy radiates out the duct inlet, but it is readily seen that the higher sound pressure level energy reflected from the untreated lower half adjacent the duct inlet radiates in a generally upward direction, while the reduced sound pressure level energy reflected from the treated upper half and out the duct inlet radiates in a generally downward direction. Thus, the directivity pattern is asymmetrical, with the preferential or enhanced noise suppression being determined by the circumferential placement of the acoustic material. These results are generally applicable to arcuate asymmetric treatments with an angular extent greater or less than 180.degree., the limits being determined at either side by practical considerations and the intended application, weighing the cost of the acoustic material versus the amount of preferential noise suppression desired. Moreover, the invention is applicable in general to acoustic ducts with cross sections other than circular, such as rectangular and square.

Referring to FIG. 4, a modification is the use of two different acoustic lining materials for optimum noise suppression effect. This is particularly well illustrated in the case of the fan duct for the aircraft jet engine. According to the modification, the upper half of the inner surface of the cowling 12 is lined with the fibrous acoustic material 20, while the lower half is lined with the previously mentioned honeycombed, resonant chamber sound-absorbing structural panel material 22. The first acoustic treatment material 22 has the advantage of durability and good drainage for ingested liquids, while the second acoustical treatment material 20 is desirably selected to obtain the combination of lower cost with a higher sound absorption coefficient. The structural acoustic panels 22 are made of, or have a facing sheet made of, a rigid material such as a suitable metal or plastic, and reflect sound more readily than the fibrous acoustic material 20. By the full treatment of the fan duct in this manner the directivity pattern as a whole is improved with good noise reduction above and forward of the duct inlet, while still retaining the enhanced noise reduction generally below the duct inlet due to the use of the better sound-absorbing material on the upper half of the duct. Instead of being circumferentially continuous as illustrated, there can be a gap between the two different acoustical treatments.

Another embodiment of the invention shown in FIG. 5 illustrates the applicability of the principle of asymmetric acoustical treatment to the suppression of side lobes. The duct 23 in this case is a hard-walled acoustical duct suitable for other applications such as a silencer for an industrial gas turbine, or in an acoustical horn structure. In the gas turbine silencer application, to explain the principle, it may be desirable to direct the noise away from residential areas. In this case, two diametrically opposing arcuate strips 24a and 24b of the same acoustic material are used. It is understood that the duct 23 has a length at least equal to or greater than the diameter and that the sound-absorbing strips 24a and 24b extend axially throughout the length of the duct or a specified portion of the length. Assuming a noise source that would produce symmetrical directivity patterns with side lobes, the effect of the asymmetric left-hand acoustic treatment 24a is to suppress the side lobe at the right-hand side of the directivity pattern, while conversely the effect of the asymmetric right-hand acoustic treatment 24b is to suppress the side lobe at the left-hand side of the directivity pattern. The explanation for the resulting altered directivity pattern, with preferential noise suppression at both sides, is similar to that for the fan duct application and need not be repeated. The required arcuate extent of the acoustic material strips 24a and 24b to produce side lobe suppression can be determined easily. As in the fan duct case (see FIG. 4), over-all noise reduction is improved by the use of two different acoustic materials, with the understanding that the side lobe suppression strips 24a and 24b are made of a material with a higher sound absorption coefficient.

In summary, asymmetric acoustic linings for suppressing sound and noise emitted from acoustic ducts, when properly located in the peripheral or circumferential direction, have the advantage of minimizing the amount of acoustic material needed for a desired result, or conversely achieve an increased effect with the same amount of treatment. The reduction of jet engine fan noise on the ground, and acoustic ducts for side lobe suppression have been discussed, but many other applications are possible.

While the invention has been particularly shown and described with reference to several preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

Claims

What is claimed is:

1. An improved noise-suppressing fan duct for an aircraft jet engine having a fan disposed within an annular fan duct air passage comprising

a generally streamlined rigid fan duct with a circular cross section having an asymmetric, circumferentially discontinuous acoustic lining attached to the inner surface thereof for absorbing noise which radiates within said duct,

said asymmetric acoustic lining being located on the upper portion of the circumference of said duct and being effective to alter the directivity of noise emitted from an end of said fan duct and produce an asymmetric directivity pattern characterized by preferential enhanced suppression of noise in an angular sector generally below the jet engine,

wherein said asymmetric acoustic lining is approximately semicylindrical and is continuous and uninterrupted in both the axial and circumferential directions, and

further including a second acoustic lining attached to the lower half of the circumference of said duct, said first-mentioned asymmetric acoustic lining having a higher sound absorption coefficient than said second acoustic lining.

2. An improved noise-suppressing fan duct for an aircraft jet engine having a fan disposed within an annular fan duct air passage comprising

a generally streamlined rigid fan duct with a circular cross section having an asymmetric, circumferentially discontinuous acoustic lining attached to the inner surface thereof for absorbing noise which radiates within said duct,

said asymmetric acoustic lining being located on the upper portion of the circumference of said duct and being effective to alter the directivity of noise emitted from an end of said fan duct and produce an asymmetric directivity pattern characterized by preferential enhanced suppression of noise in an angular sector generally below the jet engine,

wherein said asymmetric acoustic lining is made of a fibrous acoustic material and is continuous and substantially uninterrupted in both the axial and circumferential directions, and further including

a second acoustic lining attached to the lower portions of the circumference of said duct, said first-mentioned asymmetric acoustic lining having a higher sound absorption coefficient than said second acoustic lining.

3. An improved noise-suppressing fan duct for an aircraft jet engine having a fan disposed within an annular fan duct air passage comprising

a generally streamlined rigid fan duct with a circular cross section having an asymmetric, circumferentially discontinuous acoustic lining attached to the inner surface thereof for absorbing noise which radiates within said duct,

said asymmetric acoustic lining being located on the upper portion of the circumference of said duct and being effective to alter the directivity of noise emitted from an end of said fan duct and produce an asymmetric directivity pattern characterized by preferential enhanced suppression of noise in an angular sector generally below the jet engine,

wherein said asymmetric acoustic lining is made of a fibrous acoustic material, and further including

a second acoustic lining attached to the lower portion of the circumference of said duct, said second acoustic lining having provision for drainage of ingested liquids and a lower sound absorption coefficient than said first-mentioned asymmetric acoustic lining,

said first-mentioned asymmetric acoustic lining and second acoustic lining in combination being circumferentially continuous, each individual lining further being continuous and substantially uninterrupted in both the circumferential and axial directions.

4. A sound suppressing acoustic duct comprising

an axially extending, relatively rigid hard-walled duct having attached to a portion of the periphery of the inner surface thereof a first asymmetric sound absorbing acoustic lining, a second sound absorbing acoustic lining attached to the remaining portion of the periphery of the inner surface of said duct, said first and second acoustic linings in combination being peripherally continuous and functioning to absorb sound which radiates longitudinally through said duct and is reflected internally,

each of said acoustic linings individually being continuous and substantially uninterrupted in both the axial and peripheral directions, said first acoustic lining further having a higher sound absorption coefficient than said second acoustic lining,

said first asymmetric sound absorbing lining being peripherally located at any desired peripheral location to alter the directivity of sound emitted from an end of said duct and produce an asymmetric directivity pattern characterized by preferential enhanced suppression of sound in a preselected angular sector.

5. A sound suppressing acoustic duct according to claim 4 wherein said duct is an aircraft jet engine fan duct and at least said first acoustic lining is made of a fibrous acoustic material.

6. A sound suppressing acoustic duct comprising

an axially extending, relatively rigid duct having a hard-walled inner surface into which is recessed an attached acoustic lining for preferential side lobe suppression comprised by a pair of opposing sound absorbing strips for absorbing sound which radiates longitudinally through said duct and is reflected internally,

each of said sound absorbing strips being made of the same acoustic material and each being continuous and substantially uninterrupted in both the axial and peripheral directions.

said pair of opposing sound absorbing strips being peripherally spaced from one another and peripherally located and dimensioned to obtain preferential enhanced side lobe suppression of the sound emitted from an end of said duct.